docs: enforce conventional commits in `contributing.md`

[jaymanmdev]

I agree, this should go through for sure.

I've never contributed to a project before so I didn't know what the conventional commit practice was - I will make sure to adhere to it from now on!

[boywithkeyboard]

@not-ivy I'd say to enforce it for pr and issue titles. It doesn't really matter for commit messages as we squash merge them anyway.